Key Substitution in the Symbolic Analysis of Cryptographic Protocols (extended version)

Key substitution vulnerable signature schemes are signature schemes that permit an intruder, given a public verification key and a signed message, to compute a pair of signature and verification keys such that the message appears to be signed with the new signature key. A digital signature scheme is said to be vulnerable to destructive exclusive ownership property (DEO) If it is computationaly feasible for an intruder, given a public verification key and a pair of message and its valid signature relatively to the given public key, to compute a pair of signature and verification keys and a new message such that the given signature appears to be valid for the new message relatively to the new verification key. In this paper, we prove decidability of the insecurity problem of cryptographic protocols where the signature schemes employed in the concrete realisation have this two properties

Keeping Up with the KEMs: Stronger Security Notions for KEMs

Key Encapsulation Mechanisms (KEMs) are a critical building block for hybrid encryption and modern security protocols, notably in the post-quantum setting. Given the asymmetric public key of a recipient, the primitive establishes a shared secret key between sender and recipient. In recent years, a large number of abstract designs and concrete implementations of KEMs have been proposed, notably in the context of the NIST selection process for post-quantum primitives. The traditional security notion for KEMs has been the IND-CCA notion that was designed for public-key encryption (PKE). In recent work additional properties, such as robustness and anonymity, were lifted from the PKE setting to the KEMs setting. In this work we introduce several stronger security notions for KEMs. Our new properties formalize in which sense outputs of the KEM uniquely determine, i.e., bind, other values. Our new notions are based on two orthogonal observations: First, unlike PKEs, KEMs establish a unique key, which leads to natural binding properties for the established keys. Our new binding properties can be used, e.g., to prove the absence of attacks that were not captured by prior security notions, such as re-encapsulation attacks. If we regard KEMs as one-pass key exchanges, our key-binding properties correspond to implicit key agreement properties. Second, to prove the absence of weak keys, we have to consider not only honestly generated key pairs but also adversarially-generated key pairs. We define a hierarchy of security notions for KEMs based on our observations. We position properties from the literature within our hierarchy, provide separating examples, and give examples of real world KEMs in the context of our hierarchy

On the (In)Security of the BUFF Transform

The BUFF transform is a generic transformation for digital signature schemes, with the purpose of obtaining additional security properties beyond standard unforgeability, e.g., exclusive ownership and non-resignability. In the call for additional post-quantum signatures, these were explicitly mentioned by the NIST as ``additional desirable security properties\u27\u27, and some of the submissions indeed refer to the BUFF transform with the purpose of achieving them, while some other submissions follow the design of the BUFF transform without mentioning it explicitly. In this work, we show the following negative results regarding the non-resignability property in general, and the BUFF transform in particular. In the plain model, we observe by means of a simple attack that any signature scheme for which the message has a high entropy given the signature does not satisfy the non-resignability property (while non-resignability is trivially not satisfied if the message can be efficiently computed from its signature). Given that the BUFF transform has high entropy in the message given the signature, it follows that the BUFF transform does not achieve non-resignability whenever the random oracle is instantiated with a hash function, no matter what hash function. When considering the random oracle model (ROM), the matter becomes slightly more delicate since prior works did not rigorously define the non-resignability property in the ROM. For the natural extension of the definition to the ROM, we observe that our impossibility result still holds, despite there having been positive claims about the non-resignability of the BUFF transform in the ROM. Indeed, prior claims of the non-resignability of the BUFF transform rely on faulty argumentation. On the positive side, we prove that a salted version of the BUFF transform satisfies a slightly weaker variant of non-resignability in the ROM, covering both classical and quantum attacks, if the entropy requirement in the (weakened) definition of non-resignability is statistical; for the computational variant, we show yet another negative result

Seems Legit: Automated Analysis of Subtle Attacks on Protocols that Use Signatures

The standard definition of security for digital signatures---existential unforgeability---does not ensure certain properties that protocol designers might expect. For example, in many modern signature schemes, one signature may verify against multiple distinct public keys. It is left to protocol designers to ensure that the absence of these properties does not lead to attacks. Modern automated protocol analysis tools are able to provably exclude large classes of attacks on complex real-world protocols such as TLS 1.3 and 5G. However, their abstraction of signatures (implicitly) assumes much more than existential unforgeability, thereby missing several classes of practical attacks. We give a hierarchy of new formal models for signature schemes that captures these subtleties, and thereby allows us to analyse (often unexpected) behaviours of real-world protocols that were previously out of reach of symbolic analysis. We implement our models in the Tamarin Prover, yielding the first way to perform these analyses automatically, and validate them on several case studies. In the process, we find new attacks on DRKey and SOAP\u27s WS-Security, both protocols which were previously proven secure in traditional symbolic models

Backward Private DSSE: Alternative Formulations of Information Leakage and Efficient Constructions

Dynamic Searchable Symmetric Encryption ($\mathsf{DSSE}$), apart from providing support for search operation, allows a client to perform update operations on outsourced database efficiently. Two security properties, viz., forward privacy and backward privacy are desirable from a $\mathsf{DSSE}$ scheme. The former captures that the newly updated entries cannot be related to previous search queries and the latter ensures that search queries should not leak matching entries after they have been deleted. These security properties are formalized in terms of the information leakage that can be incurred by the respective constructions. Existing backward private constructions either have a non-optimal communication overhead or they make use of heavy cryptographic primitives. Our main contribution consists of three efficient backward private schemes that aim to achieve practical efficiency by using light weight symmetric cryptographic components only. In the process, we also revisit the existing definitions of information leakage for backward privacy [Bost et al. CCS\u2717] and propose alternative formulations. Our first construction $\Pi_\mathsf{BP}\text{-}\mathsf{prime}$ achieves a stronger notion of backward privacy whereas our next two constructions $\Pi_\mathsf{BP}$ and $\Pi_\mathsf{WBP}$ achieve optimal communication complexity at the cost of some additional leakage. The prototype implementations of our schemes depict the practicability of the proposed constructions and indicate that the cost of achieving backward privacy over forward privacy is substantially small

Digital Signatures Do Not Guarantee Exclusive Ownership

Abstract. Digital signature systems provide a way to transfer trust from the public key to the signed data; this is used extensively within PKIs. However, some applications need a transfer of trust in the other direction, from the signed data to the public key. Such a transfer is cryptographically robust only if the signature scheme has a property which we name exclusive ownership. In this article, we show that the usual signature algorithms (such as RSA[3] and DSS[4]) do not have that property. Moreover, we describe several constructs which may be used to transform a signature scheme into another signature scheme which provides exclusive ownership.